steam pressure of 15 Ib. and a vacuum of 20 in., the capacity is from 1 to 1 -Hz gal. per square foot. Single effects for small quantities; double effects operated counter-current for larger volumes. Evaporators are made of cast iron with wrought-iron or copper tubes. Natural epsom-salt solutions will deposit a scale of sodium sulphate on the tubes, and the solutions produced from dolomite and sulphuric acid will form a heavy scale of calcium sulphate. Tubes must therefore be accessible for mechanical cleaning.
Potassium Bichromate.—Concentrated from 25 to 50 per cent in a vertical-tube evaporator with a steam pressure of 5 Ib. and a vacuum of 26 in., at a rate of 1 gal. per square foot. Sodium chloride is separated in salt filters. Evapora-* tors must be built of cast-iron shells and charcoal-iron or steel tubes.
Potassium Carbonate.—Density of weak liquor will vary greatly, dependins on the source of the product. Solutions are concentrated until the salt crystal, separate in a vertical-tube evaporator with a steam pressure of from 5 to 10 Ib and a vacuum of 26 in., at a capacity of 1 gal. per square foot. The carbonate crystals are recovered in salt niters. Evaporators are made of cast iron or steel, and tubes of steel or charcoal iron.
Potassium Chloride.—Density of weak liquor solution will depend on the source of material. It will vary from 5 to 20 per cent of solids. It is usually concentrated up to the crystallization point, and crystals are recovered by cooling in crystallizing vats. Capacity will be from 1 to 1H gal- per square foot, steam pressure of 5 Ib. and a vacuum of 26 in. Horizontal-tube or rapid-c tion type evaporators may be used, and vertical-tube evaporators must be Ubo^ for liquors that also separate sodium chloride. Cast-iron shells with either charcoal-iron or copper tubes are used.
Potassium Hydroxide.—Usually concentrated from 10 to 46 per cent in multiple-effect evaporators of vertical-tube construction, with a steam pressure of from 5 to 20 Ib. and a vacuum of from 26 to 28 in., at a capacity of about 1 gal. per square foot. Potassium carbonate or chloride are recovered in salt filters. Evaporator shells are of cast iron and tubes of charcoal iron.
Potassium Nitrate.—Concentrated from 30 to 70 per cent in a vertical-tube evaporator with a capacity of 1 gal. per square foot, and a steam pressure of 15 Ib. and a vacuum of 27 in. A double effect operating counter-current is advisable, and the sodium chloride is recovered in salt filters. Evaporator bodies are of cast iron and tubes of copper or charcoal iron.
Potassium Sulphate.—The weak liquors containing from 10 to 20 per cent of sulphate crystals are concentrated in vertical-tube evaporators with a steam pressure of about 5 Ib. and a vacuum of 27 to 28 in., at a capacity of 1 to 1J^ gal. per square foot. The sulphate crystals are recovered in salt filters attached to the evaporators, which are built of either cast-iron or steel with steel or charcoal-iron tubes.
Sodium Bichromate.—Liquors containing from 50 per cent bichromate are concentrated for the separation of the sodium chloride in vertical-tube evaporators, with a steam pressure of 5 Ib. and a vacuum of 26 in. at a capacity of 0.8 gal. per square foot. The sodium chloride is recovered in salt filters. Multiple effects should be operated counter-current, and evaporators can be built of cast iron or steel with steel or charcoal-iron tubes.
Sodium Carbonate.—The weak solutions containing about 10 per cent of